The present invention concerns the manufacture of uranium dioxide powder by the ammonium uranyl carbonate method, also called the AUC method, in which UF.sub.6 is first transformed into the gaseous phase and the latter is hydrolyzed in a water receiver with the simultaneous addition of NH.sub.3 and CO.sub.2 ; the AUC precipitated thereby is separated from the mother liquor by filtering and washing, and the UO.sub.2 is formed by decomposition and reduction of the AUC at 500.degree.C in a water vapor/hydrogen atmosphere. The uranium dioxide powder prepared in this manner is put into pellet form, for instance, by pressing and sintering and is enclosed as nuclear fuel in the fuel cladding tubes of nuclear reactor fuel elements. This manufacture of uranium dioxide is described in detail in German Pat. Nos. 1,126,363; 1,592,477 and 1,924,594.
The thermal decomposition of the AUC and the reduction of the hexavalent oxide into UO.sub.2 in hydrogen gas is normally carried out in a furnace or in two separate furnace units, such as so-called vortex-bed furnaces.
The thermal decomposition of the AUC is an endothermic process step which, together with the adhering moisture, limits the capacity of any furnace, as a substantial part of the heat that is to be supplied by the furnace to the powder is first required to evaporate the residual water. In order to increase the throughput of these vortex-bed furnaces, it would, therefore, be desirable to reduce the moisture content of the AUC as much as possible.